The present invention relates to fluid control systems, and in particular to electromechanical valves for controlling hydraulic and pneumatic equipment.
The field of hydraulics involves the movement of fluidsxe2x80x94liquids and gasesxe2x80x94through systems for various purposes. Hydraulics are used in a wide range of applications including gas-station pumps, control surfaces in aircraft, fuel-injection systems, and theme-park style animatronics. Hydraulic systems require control over inlet pressure and flow to hydraulic components (such as pistons). Because of the high pressures involved, the need for precision, and the desirability of operating over a range of action frequencies, the cost of hydraulic control systems can be quite high. Obtaining reasonable operating bandwidth generally favors the use of hydraulic servovalves, which are the most costly of all.
More generally, a hydraulic control valve is a device that uses mechanical motion to control a source of fluid power. A widely used design is a sliding valve employing a spool-type construction, in which a valve stem having a series of raised portions, or lands, moves within the bore of a cylindrical housing. Radial ports deliver fluid into and conduct fluid from the bore. The stem lands slide along the interior bore surface in the manner of a piston head as the valve stem is reciprocated within the bore, and each land is capable of limiting or completely blocking fluid flow through one of the ports.
Accordingly, the particular combination of inlet and outlet ports operative. at any one time is controlled by the position of the valve stem. If the width (i.e., the axial extent) of the of a land is smaller than the port diameter, the valve is said to have an open center or to be xe2x80x9cunderlapped.xe2x80x9d In a xe2x80x9ccritical centerxe2x80x9d or xe2x80x9czero-lappexxe2x80x9d valve, the land width is identical to the port diameter (a condition approached by practical machining). Closed-center or overlapped valves have land widths that exceed the port diameter.
In hydraulic systems operating in high-pressure, high-flow environments, the valve stem can experience substantial resistance to movement due to pressure differences between inlet and outlet ports. Accordingly, the mechanism used to drive the stem must be capable of generating the necessary force with sufficient speed to accommodate performance requirements. Traditionally, movement of the stem has been accomplished by means of an armature extending from the stem to a drive system disposed outside the housing. This configuration requires a fluid seal, resulting in both high cost and unreliability.
Solenoid-type systems utilize magnetic force to drive the valve stem, and the solenoid elements themselves can be made sufficiently small to fit within an extended housing. Such systems are exemplified by the disclosures of U.S. Pat. Nos. 5,106,053 and 5,460,201. The servovalves disclosed in these patents once again utilize an armature arrangement, but the armature is reciprocated by means of a solenoid contained within the housing. Although such designs avoid the need for fluid seals, they are still fundamentally transmission arrangements: the motive force is generated by an element mechanically distinct from the valve stem, and must be transmitted to the stem by means of a rigid element. As a result, these valves are mechanically complex and costly to manufacture.
In accordance with the present invention, the valve stem itself is made a part of a solenoid arrangement that effects its reciprocation. The valve stem is magnetically responsive (e.g., magnetically permeable or ferromagnetic), and a magnetic field is applied to the stem to cause it to move within the bore of the valve housing; there is no external element required to move the valve stem. The valve stem is typically sealed permanently within the bore.
A coil concentrated or disposed toward one end of the valve housing is capable of drawing the valve stem in that direction. Movement in the opposite direction can be obtained in various ways. In one approach, a second coil is similarly concentrated or disposed toward the opposite end of the valve housing; this coil can act to draw the valve stem or to brake movement effected by the first coil. In another approach, the opposite end of the valve stem is connected to a spring or other means urging the valve stem in the opposite direction. And in still another approach, the valve stem comprises a permanent magnet, and the orientation of the magnetic field applied by a coil is reversed to change the direction of movement.
The valve is useful in both hydraulic and pneumatic applications. In a hydraulic application, the bore in which the valve stem moves is typically filled with hydraulic liquid, the incompressibility of which can also be used as a braking mechanism. For example, the valve housing may comprise one or more ducts extending along the exterior housing surface between two apertures, one leading into the bore and the other leading to a fluid reservoir (e.g., via an outlet vent). A land reciprocates in the vicinity of a duct such that its point of maximum excursion does not reach beyond the aperture into the bore. With the duct open, the valve stem moves freely since fluid between the land and the aperture may be displaced through the duct into the outlet port. If the duct is closed (e.g., by means of a needle valve), on the other hand, the incompressible hydraulic fluid within the bore prevents valve-stem movement.